The supercontinuum light source (SC light source), which is a kind of broadband light source, has characteristics such as high output performance, broadband performance, spectral flatness, etc. Such a broadband light source is expected as an important light source to be applied to various fields: for example, application to optical measurement and near infrared spectroscopy. Of various compositions proposed as such a broadband light source, generally the broadband light source that can generate SC light in an optical fiber is widely used, since the composition is simple, and it is easy to elongate the interaction length and to control the spectrum.
On the other hand, main examples of light sources which are used as a seed light source for emitting seed light to be input into an optical fiber in order to generate SC light are a short pulse light source on the order of picosecond or femtosecond, a pulsed light source on the order of nanosecond, a continuous wave light source (CW light source), etc. A typical example of the short pulse light source is a titanium sapphire laser beam source, which is widely used in general. An example of a light source to oscillate in the vicinity of the 1550 nm wavelength, which is an important wavelength band particularly in optical communications, is a fiber laser beam source including an erbium-doped optical fiber as an optical amplification medium. Such a fiber laser beam source is also under active development as a light source for generating short pulse light.
A number of reports regarding the generation of SC light have been published in the past. Recently, active investigations are conducted using a holey fiber such as a photonic crystal fiber which has a zero dispersion wavelength in the vicinity of the oscillation wavelength of a titanium sapphire laser beam source. The SC light generated with a holey fiber can achieve a very broad band of about 400 nm to 1700 nm. However, a number of problems are involved: for example, the spectrum of the SC light is unstable because of a large polarization mode dispersion (PMD) of the holey fiber, and the pulsed light source system itself is large-sized, resulting in difficulty of handling, etc.
On the other hand, the short pulse light source of 1550 nm wavelength band, a typical example of which is an Er-doped fiber laser beam source, has merits in terms of stability, small size, and portability, and is advantageous in that high output can easily be obtained if it is used in combination with an optical amplifier. According to certain reports, by combining this light source with a highly nonlinear optical fiber, SC light of broad bandwidth having a wavelength band of 1140 nm to 2400 nm is generated. (Refer to non-patent documents 1 and 2).
However, in the past, it has been difficult to simultaneously expand the spectrum of the SC light to 2200 nm or more on the longer wavelength side and to 1100 nm or less on the shorter wavelength side, and in fact there is little report on such an example. A technique intended to further expand the broad bandwidth of SC light is disclosed in a non-patent document 3. In the technique disclosed in this non-patent document 3, it is attempted that the expansion of the SC light spectrum be achieved by irradiating ultraviolet light to the optical fiber so as to shift the zero dispersion wavelength of an optical fiber to the shorter wavelength side. However, there are problems yet to be solved in the technique: large-scale irradiation equipment is necessary to irradiate ultraviolet light to the optical fiber; the shifting is irreversible and accordingly the adjusting of fiber characteristics is difficult, etc. Thus, it has been difficult in the past to achieve a light source that can output SC light having a broad bandwidth covering a range of 1.0 μm to 2.3 μm (further, a wavelength range of 0.8 μm to 2.5 μm), although it would be useful if such a light source is available.
Non-patent document 1: T. R. Schibli, et al.: Opt. Lett. Vol. 29 (2004) 2467.
Non-patent document 2: Okuno, et al.: The brief collection of lectures at 21st Near-infrared Forum, P-33, Page 173 (2005).
Non-patent document 3: P. S. Westbrook, et al.: J. of Lightwave Techn. Vol. 23 (2005) 13.